Flotation is a clarification technology (solid-liquid separation) which offers an alternative to settling, at least for certain types of water.
According to this known technology (see in particular “Memento Technique de l'Eau” 1989, Volume 1, pages 171 et seq.), after a coagulation-flocculation step, the water is mixed with a “milk” (that is, an emulsion) of microbubbles, generally of air, the average diameter of which is between 40 and 80 microns. These microbubbles stick to the flocs which, thereby lightened, tend to rise to the surface of the flotation cell, where they accumulate to form a sludge layer or bed. The sludges are collected at the surface of the flotation unit, while the clarified water is removed via the bottom of the apparatus.
A portion of this water is pumped (delivery generally between 5 and 15% of the water flow rate to be treated by clarification), at a pressure of about 4×105 to 6×105 Pa into a specific tank, called pressurization tank, in which the air is dissolved in large amounts, that is, up to five times the maximum concentration of air in water at atmospheric pressure. During a sudden expansion to atmospheric pressure, the air is placed in a condition of supersaturation and generates microbubbles. The expansion systems are placed in a specific zone in which the microbubbles are mixed with the flocculated water.
To be physically separated from the water in a settler, a floc must be dense and large sized. On the contrary, to be separated by flotation, it suffices for said floc to be properly formed: it can be light and small sized. Flocculation can therefore be simplified, hence the virtually general absence of use of polymer for the flotation treatment of relatively uncontaminated waters, and the use of smaller reactors than in the case of settling units situated downstream of a diffuse flocculation (as opposed to the sludge bed or “ballasted” settlers).
The weak point of flotation is that the microbubbles stick with difficulty to the mineral particles and cannot guarantee that the heavy particles present in the water will rise to the surface. Accordingly, the applications of flotation are often limited to the clarification of relatively uncontaminated waters, particularly lake waters, wells, seawater and specific industrial effluents or wash waters from biological filters.
The other features and advantages of flotation include the following in particular:                the pressurization system is very simple and it is very rapidly ramped up to operating regime. Flotation units start nearly instantaneously: they are very simple units to operate, even in intermittent operation;        the sludges extracted are concentrated: up to 10 to 40 g/l, if they are scraped;        the microbubbles have upflow velocities of 6 to 12 m/h, which results in clarification rates conventionally limited between 4 and 10 m/h.        
Despite their advantages, flotation units have hardly been able to compete with the generation of high-speed lamellar settlers, with sludge bed or ballast, particularly for the following reasons:                generally oversized volume of the flocculation zone;        relatively low separation rates;        pressurization energy cost and        relatively limited field of application.        
However, high-speed flotation units have appeared in recent years, thanks to the use of countercurrent lamellar modules or specific retrieval systems (EP 0 659 690). According to these new techniques, clarification rates in the range of 20 to 40 m/h can be obtained. Moreover, flocculation studies have shown that static or hydraulic flocculators (equipped with a series of deflectors and baffles), by achieving a plug-flocculation, help to halve the time required for flocculation by stirrer, for example, in certain cases, from ten minutes to five minutes. FIG. 1 of the drawings appended hereto shows an embodiment of a flotation unit employing this technique. In this figure, the numeral 10 designates the coagulator, the numeral 11 the flocculator comprising a series of deflectors and baffles, and the numeral 12 the flotation cell. The pressurization tank is designated by the numerals 13. The suspended matter accumulating at the surface of the flotation cell is removed by a surface scraper system 14 and a sludge discharge at 15, the clarified water being removed at 24.
In such a known installation, which offers short flocculation times and high rates in the flotation unit 12, flotation can become extremely competitive with settling: today, the aim of a person skilled in the art is to design flotation units in which the flocculation time is about 5 minutes, with separation rates of 30 to 40 m3/m2·h.
Flotation technology is accordingly making a strong comeback in connection with the clarification of low-contaminated waters, given the competitive costs of this technology compared with settling, and also because of its obvious simplicity of operation.
By contrast, the major drawback of flotation is that this technique cannot extend its field of application to the vast field of river waters, wastewaters (primary, rainfall, etc.), wash waters, etc. due to the difficulty, indeed the impossibility, of “floating” dense and/or large-sized particles. Attempts have nonetheless been made to design flotation units which can operate on these difficult waters. However, the results obtained are very mediocre, both in terms of operating cost and treatment quality. This entails the need to use stirring systems such as propellers to prevent deposits in the flocculators and to provide a bottom scraper system in the flotation cell. FIG. 2 in the drawings appended hereto shows an embodiment of this type of installation. It shows at 16 the propellers positioned in the flocculator 11 and at 17 the scraper placed at the bottom of the flotation cell 12. The major drawback of this type of flocculation by mechanical stirring is that it imposes bulky flocculators and “inflates” the residence times of the effluent to be treated in the installation.
Furthermore, the settling of heavy flocs at the bottom of the flotation cell 12 (where the outlet of the treated effluent is located) and their re-placement in suspension during the passage of the scraper 17 produces a mediocre quality clarified water.